diff --git a/README b/README
index 191b7c7..36f9e65 100644
--- a/README
+++ b/README
@@ -88,6 +88,10 @@ directory from package.cpath).
     iconv.ERROR_INCOMPLETE
         An incomplete character was found in the input sequence.
 
+    iconv.ERROR_FINALIZED
+        Trying to use an already-finalized converter. This usually means
+        that the user was tweaking the garbage collector private methods.
+
     iconv.ERROR_UNKNOWN
         There was an unknown error.
 
diff --git a/luaiconv.c b/luaiconv.c
index f54a621..e2e0697 100644
--- a/luaiconv.c
+++ b/luaiconv.c
@@ -53,6 +53,7 @@
 #define ERROR_INVALID       2
 #define ERROR_INCOMPLETE    3
 #define ERROR_UNKNOWN       4
+#define ERROR_FINALIZED     5
 
 
 
@@ -66,9 +67,7 @@ static void push_iconv_t(lua_State *L, iconv_t cd) {
 static iconv_t get_iconv_t(lua_State *L, int i) {
     if (luaL_checkudata(L, i, ICONV_TYPENAME) != NULL) {
         iconv_t cd = UNBOXPTR(L, i);
-        if (cd == (iconv_t) NULL)
-            luaL_error(L, "attempt to use an invalid " ICONV_TYPENAME);
-        return cd;
+        return cd;  /* May be NULL. This must be checked by the caller. */
     }
     luaL_argerror(L, i, lua_pushfstring(L, ICONV_TYPENAME " expected, got %s",
         luaL_typename(L, i)));
@@ -100,6 +99,12 @@ static int Liconv(lua_State *L) {
     size_t ret = -1;
     int hasone = 0;
 
+    if (cd == NULL) {
+        lua_pushstring(L, "");
+        lua_pushnumber(L, ERROR_FINALIZED);
+        return 2;
+    }
+
     outbuf = (char*) malloc(obsize * sizeof(char));
     if (outbuf == NULL) {
         lua_pushstring(L, "");
@@ -182,8 +187,13 @@ static int Liconvlist(lua_State *L) {
 
 static int Liconv_close(lua_State *L) {
     iconv_t cd = get_iconv_t(L, 1);
-    if (iconv_close(cd) == 0)
+    if (cd != NULL && iconv_close(cd) == 0) {
+        /* Mark the pointer as freed, preventing interpreter crashes
+           if the user forces __gc to be called twice. */
+        void **ptr = lua_touserdata(L, 1);
+        *ptr = NULL;
         lua_pushboolean(L, 1);  /* ok */
+    }
     else
         lua_pushnil(L);         /* error */
     return 1;
@@ -207,6 +217,7 @@ int luaopen_iconv(lua_State *L) {
     TBL_SET_INT_CONST(L, ERROR_NO_MEMORY);
     TBL_SET_INT_CONST(L, ERROR_INVALID);
     TBL_SET_INT_CONST(L, ERROR_INCOMPLETE);
+    TBL_SET_INT_CONST(L, ERROR_FINALIZED);
     TBL_SET_INT_CONST(L, ERROR_UNKNOWN);
 
     lua_pushliteral(L, "VERSION");
diff --git a/test_iconv.lua b/test_iconv.lua
index 972778f..a70b0d0 100644
--- a/test_iconv.lua
+++ b/test_iconv.lua
@@ -97,3 +97,14 @@ check_one(termcs, "utf8", utf8)
 check_one(termcs, "utf16", utf16)
 check_one(termcs, "EBCDIC-CP-ES", ebcdic)
 
+
+-- The library must never crash the interpreter, even if the user tweaks
+-- with the garbage collector methods.
+local cd = iconv.new("iso-8859-1", "utf-8")
+local _, e = cd:iconv("atenção")
+assert(e == nil, "Unexpected conversion error")
+local gc = getmetatable(cd).__gc
+gc(cd)
+local _, e = cd:iconv("atenção")
+assert(e == iconv.ERROR_FINALIZED, "Failed to detect double-freed objects")
+gc(cd)